The surface of grain-oriented electrical steel sheet is formed with an insulating film comprised of the two layers of a forsterite film called a “primary film” formed after cold rolling and decarburizing annealing during high temperature final annealing and a phosphate film formed by coating and baking a treatment solution mainly comprised of a phosphate etc. after the final annealing at the same time as the flattening.
The forsterite film plays an important role in improving the adhesion of the steel sheet and phosphate film.
The phosphate film is a film required for imparting a high electrical insulating ability to the grain-oriented electrical steel sheet and reducing the eddy current loss to improve the watt loss. The above film is being asked to provide, in addition to an insulating ability, various properties such as adhesion, heat resistance, slip, and corrosion resistance.
When working grain-oriented electrical steel sheet to produce a core of a transformer etc., if the film is degraded in adhesion, heat resistance, or slip, the film will peel off at the time of the stress-relief annealing whereby the inherent performance of the film will not be expressed or the steel sheet will not be able to be smoothly stacked and the work efficiency will be degraded.
If using an insulating film to impart tensile strength to the surface of electrical steel sheet, movement of the magnetic domain walls becomes easier and as a result the watt loss is reduced and the magnetic properties are improved. Imparting tensile strength is also effective for reducing the magnetostriction—which is one of the main causes of transformer noise.
Japanese Patent Publication (B2) No. 53-28375 discloses a method of coating a forsterite film formed on surface of steel sheet after final annealing with an insulating film treatment solution mainly comprised of a phosphate, chromate, and colloidal silica and baking it to form a high tensile strength insulating film and thereby reduce the watt loss and magnetostriction.
Further, Japanese Patent Publication (A) No. 61-41778 discloses a method of coating a treatment solution containing superfine particles of colloidal silica having a particle size of 8 μm or less, a primary phosphate, and a chromate in specific ratios and baking it on to hold the tensile strength of the insulating film at a high tensile strength level and improve the lubricating ability of the film.
Furthermore, Japanese Patent Publication (A) No. 11-71683 discloses the technology relating to grain-oriented electrical steel sheet having a high tensile strength mainly comprised of a phosphate, chromate, and colloidal silica having a glass transition point of 950 to 1200° C.
According to the technology disclosed in the above publications, various types of film properties are remarkably improved and, further, the film tensile strength is also improved, but the insulating film contains the chrome compound of a chromate.
In recent years, environmental issues have come into the spotlight. Use of compounds of lead, chrome, cadmium, etc. is being prohibited or restricted. Therefore, technology not using chrome compounds is being sought.
As the above art, Japanese Patent Publication (B2) No. 57-9631 discloses the method of baking a treatment solution containing colloidal silica in an amount, by SiO2, of 20 parts by weight, aluminum phosphate in an amount of 10 to 120 parts by weight, boric acid in an amount of 2 to 10 parts by weight, and one or more sulfates of Mg, Al, Fe, Co, Ni, and Zn in an amount of 4 to 40 parts by weight at a temperature of 300° C. or more to form an insulating film.
Furthermore, Japanese Patent Publication (A) No. 2000-178760 discloses technology relating to a surface treatment agent for grain-oriented electrical steel sheet containing, as an organic acid salt selected from Ca, Mn, Fe, Zn, Co, Ni, Cu, B, and Al, one or more organic acid salts selected from formates, acetates, oxalates, tartarates, lactates, citrates, succinates, and salicylates.
However, the method disclosed in Japanese Patent Publication (B2) No. 57-9631 has the problem of a drop in the corrosion resistance due to the sulfate ions in sulfates. Further, the technology disclosed in Japanese Patent Publication (A) No. 2000-178760 has a problem relating to solution stabilization, that is, discoloration due to organic acids in the organic acid salts. Further improvement is necessary.
Further, Japanese Patent Publication (A) No. 1-147074 discloses grain-oriented silicon steel sheet provided with an insulating film mainly comprised of a phosphate and colloidal silica in which local regions with large crystallinity degrees are formed.
The insulating film of the grain-oriented silicon steel sheet disclosed in Japanese Patent Publication (A) No. 1-147074 has regions with large crystallinity degrees locally formed in the film, so effectively gives tensile strength to the steel sheet and as a result achieves a reduction in the watt loss.
However, in the above publications, the adhesion of the insulating film is not evaluated. The adhesion of the insulating film is believed to be that of the conventional level. In this respect, the insulating film disclosed in the above publication has room left for improvement.
Japanese Patent No. 348237 discloses assisting the phosphoric acid freed from the hydrogen phosphate in the first layer by adding free phosphoric acid to that first layer and, when adding free phosphoric acid in excess and the amount of phosphoric acid in the first layer becomes in excess, jointly using chromium oxide, thereby not only improving the corrosion resistance, but also preventing sticking at the time of stress-relief annealing by the excess phosphoric acid.
However, the technology disclosed in the above publication requires a second layer mainly comprised of aluminum borate and takes note of the chemical affinity between free phosphoric acid and the second layer. It requires a layered structure comprised of a plurality of layers (first layer and second layer), so has the problem industrially of the cost becoming higher.